1. Field of the Invention
The present invention relates to an image reader, which is generally called a scanner, for reading an image on an original by optically scanning the original and outputting data of the read image as electric signals. The image reader is usually used aboard a digital copying machine, a facsimile machine, a color scanner or the like.
2. Description of Related Art
Conventional color image readers generally scan an original placed on a transparent original platen mainly composed of glass with light emitted from a light source and receive light reflected from the original to read an image on the original. For reading an color image, the image readers resolve the reflected light by some means into three components of light, i.e., R(red), G(green) and B(blue), and direct the three components on a light-receiving section of a photoelectric converter such as a CCD (charge-coupled device), which serves as a reading element, so as to convert them into analog electric signals of the three components R, G and B. The signals are then outputted in the form of digital electric signals by means such as an A/D converter.
In the above-described image readers, there arises a phenomenon called shading distortion owing to various causes such as variations in quantity of light from the light source, variations in conversion characteristics of photoelectric conversion elements defining pixels, changes in quantity of light from the light source caused by changes of voltage in the device, changes in the conversion characteristics of the photoelectric conversion elements with time, and variations in quantity of light passing through a lens for forming images. The shading distortion means that the electric signals outputted by the photoelectric converter vary and do not represent a constant value even if an image of a consistent shade of color is taken by the image reader through optical scanning and directed to the photoelectric converter.
Various attempts have been made for the correction of the shading distortion to minimize variations in outputted signals and always obtain constant electric signals, regardless of the above-mentioned causes, when reading an image of the consistent shade. Such correction is called shading correction.
A generally employed method for shading correction is setting a reference white sheet for white balance (hereafter referred to simply as white balance sheet) in a place other than the original platen on a travel path of an optical system which optically scans and reads an image on an original. This white balance sheet is optically scanned and the image thereof is read by the photoelectric converter before an original is optically scanned. On the basis of data of this read image, correction values for the signals representative of the R, G and B components are calculated for every photoelectric conversion element defining a pixel. Data of the read original is corrected by adding the calculated correction values.
As one example of such system, Japanese Unexamined Patent Publication No. Hei 7(1995)-336535 discloses a method for stable shading correction by setting a white balance sheet with bar codes representing colorimetric data for all photoelectric conversion elements, and reading the bar-code data and optically scanning the white balance sheet before reading the original, thereby to temper variations in shade of color which occur in the production of the reader.
Japanese Unexamined Patent Publication No. Hei 8(1996)-172531, on the other hand, discloses a method in which a white balance sheet is arranged on a scanning line on a original platen, and shading correction values for sites where the white balance sheet is not located are calculated from signal values of the white balance sheet by linear estimation.
The above-described methods for the shading correction have some effect in their own ways. That is to say, data of an image actually read can be outputted in a consistent shade by the correction of the data in consideration of the data outputted with regard to the white balance sheet.
However, the shading correction disclosed by Japanese Unexamined Patent Publication No. Hei 7(1995)-336535 cannot reduce variations in outputted signals all over a face for reading originals, though it can reduce variations in outputted signals among the photoelectric conversion elements to some extent. More particularly, when an original on the original platen is optically scanned, owing to such causes as changes in quantity of light from the light source and changes in angle of scanning light directed by the light source onto the scanning lines, images of the original which have the same shade may be represented by signals having different values depending on where on the original the images are located.
Also in the shading correction disclosed by Japanese Unexamined Patent Publication No. Hei 8(1996)-172531, it is extremely difficult to perform an accurate shading correction on sites where the white balance sheet is not located.
Therefore, by any of the shading correction methods or systems, it is very difficult to always perform a satisfactory correction to the entire original, and the shading distortion always occurs to some extent depending on in what part of the original platen the image to be read is located. Further, the shading correction methods require the reading of the white balance sheet every time before reading an image on an original, in order to perform shading correction on the basis of the white balance sheet. As a result, time necessary for reading tends to be long.